The possibility of recovering fetal cells from maternal blood has long been a goal for geneticists interested in prenatal diagnosis. This would extend prenatal diagnosis (at least for cytogenetic disorders) to the entire population, not merely those at increased risk. In 1979, Hertzenberg and colleagues used flow-sorting techniques to recover lymphocytes having a paternal but not maternal HLA-A2 antigen, thus presumably being fetal in origin. However, considerable skepticism persisted because confirmation of fetal origin was based on Y chromatin analysis. After nearly a decade of ostensible inactivity, several groups have recently used polymerase chain reaction (PCR) to show that fetal cells very likely exist in maternal blood. The first group to do so was that of Lo et al., who used nested primers for a Y sequence. Women carrying male fetuses proved far more likely to show a Y-specific signal than those carrying female fetuses. The candidate cell our University of Tennessee group as well as others are pursuing is the nucleated fetal red cell (erythroblast.) We thus are employing fluorescence in situ hybridization (FISH) with chromosome-specific DNA probes. In one of our first successes, a blood sample taken prior to CVS and flow-sorted revealed trisomy 18 cells. Using various enrichment strategies to isolate nucleated red cells in combination with FISH, others have subsequently detected fetal aneuploidies also. In this conference, we will review the scientific basis of fetal cell recovery, and current state of clinical application. All active workers seeking to recover the various cell types will be invited. We shall explore the scientific basis underlying passage of fetal cells (trophoblasts, lymphocytes, nucleated red blood cells) into the maternal circulation, the scientific basis of flow sorting and other enrichment techniques, the pitfalls and potential of in situ hybridization for diagnosis. and ethical and logistical requirements before clinical introduction. The conference shall begin with a general overview of historical aspects. Next, we shall consider placenta development, including physiological and anatomical developments that could favor passage in the fetal circulation of one cell type versus another. Immunologic signals between mother and fetus will be considered as a clue for explaining the phenomena of fetal cells in maternal blood. The current status of work by the 10-12 groups seeking to recover fetal cells will be reviewed. In-depth consideration will then focus on two areas in which fundamental advances appear pivotal to clinical application. One involves enrichment for rare cells, namely by either flow sorting or magnetic cell separation. The other is in situ hybridization (ISH), especially multi- color techniques. We will then consider basic technology underlying flow sorting, use of fluorescent probes and, finally, the application of in situ hybridization to flow sorted cells. Underlying our discussion will be assessment of the sensitivity and specificity of this technology in the general population. Overall, we expect this conference to update current status in the field, help identify the most profitable of several potential approaches and allow formulation of collaborative studies that will include exchange of samples to determine sensitivity and specificity.